1. Field of the Invention
Embodiments disclosed herein generally relate to a structure for a perpendicular magnetic recording head used in a magnetic disk device.
2. Description of the Related Art
Investigations have been carried out in recent years into microwave-assisted magnetic recording (MAMR) as a recording method for increasing recording density. In MAMR, an AC magnetic field from a spin-torque oscillator (STO) is applied to a medium in addition to the magnetic field from a main pole. When the AC field is applied to the medium, the coercive force of the medium decreases, and high-quality recording is readily achieved. It is therefore important in MAMR to develop an STO that can generate a sufficiently large AC field. The STO 100 in FIG. 1 comprises a field generation layer (FGL) 102 for generating an AC field, an interlayer or spacer 104, and a spin polarization layer (SPL) 106 for transmitting spin-polarized torque.
A material having strong perpendicular anisotropy energy is used for the SPL 106. Furthermore, the STO 100 is energized with current in the direction from the SPL 106 to the FGL 102 as shown by arrow “A”. In this process, spin torque in the same direction as the magnetization of the FGL 102 acts on the magnetization of the SPL 106, and spin torque acts on the magnetization of the FGL 102 in a direction which is anti-parallel with the magnetization of the SPL 106. Furthermore, a perpendicular magnetic field is applied to the STO 100, and therefore the magnetization of the SPL 106 is stable in the perpendicular direction as shown by arrow “B”. On the other hand, the FGL 102 magnetization shown by arrow “C” oscillates with a large in-plane component 108. Oscillation of the STO 100 in this structure is referred to as T-mode oscillation because the SPL 106 and FGL 102 oscillate in a “T”-shape.
As shown in FIG. 2, the STO 200 structure also comprises an FGL 102 for generating an AC field, an interlayer or spacer 104, and an SPL 106 for transmitting spin-polarized torque. The difference in FIG. 2 lies in the fact that the magnetization of the SPL 106 is effectively in the film in-plane direction as shown by arrow “D”, and both the FGL 102 and SPL 106 oscillate. According to the specific structure which is used, an energizing current flows in the direction from the FGL 102 to the SPL 106 as shown by arrow “E”, and the SPL 106 is thin and the perpendicular anisotropy field is low, so that the effective anisotropy field of the SPL 106 is zero. A feature of this structure lies in the fact that there is no delay in reversal of the magnetization of the SPL 106 due to switching of the current polarity of the write head field, so reversal of the FGL 102 also occurs rapidly which is advantageous for high-speed transfer recording. When the STO 200 oscillates, a state in which the SPL 106 and FGL 102 are anti-parallel is maintained, and is therefore referred to as antiferromagnetic-mode (AF-mode) oscillation.
A feature required of an STO is to increase the generated AC field, and to this end it is effective to increase the spin torque acting on the FGL. The magnitude of the spin torque is proportional to the density of the current to the STO, and therefore a high AC field intensity can be achieved by increasing the applied current. FIG. 3 shows the relationship between AC field intensity and amount of current applied in an STO structure where the FGL film thickness is 7.5 nm and 15 nm. FIG. 4 and FIG. 5 show the state of magnetization when an STO 400, 500 having an FGL 102 with a film thickness of 7.5 nm and 15 nm, respectively, is seen from the surface opposite the medium. The state of magnetization is the result of a numerical value calculation in a micro-magnetic simulation. The energizing current to the STO 400, 500 is 9 mA. It is clear from FIG. 3 that the AC field intensity is increased as a result of an increase in the energizing current to the STO, and saturation is reached. Furthermore, the saturation value of the AC field intensity is largely unchanged for an STO in which the FGL film thickness is 7.5 nm or 15 nm. This is because, as is clear from FIG. 4 and FIG. 5, the in-plane component of the FGL magnetization is attenuated by a greater amount further away from the spacer interface. This can be explained by the fact that the FGL magnetization distribution shown by arrows “F” and “G” is believed to become weaker further away from the spacer interface while the spin torque acting on the FGL 102 is most intense in the region of the spacer interface, as seen in FIG. 5. Furthermore, saturation of the AC field intensity means that the spin torque of the FGL 102 is excessively intense in the region of the spacer interface, so the FGL magnetization is disturbed and multiple magnetic domains are produced, whereas the increase in the film-plane component of the FGL magnetization is small at a position remote from the spacer interface. This means that a simple increase in FGL film thickness cannot be considered an effective solution for increasing the AC field intensity.
Therefore, there is a need in the art for an STO having an increased AC field intensity.